1. Field of the Invention
The present invention generally relates to the art of machining, and more specifically to a broaching method, tool and lubricant/coolant for forming small, deep holes with high precision and surface finish.
2. Description of the Related Art
Machining of small, deep holes with high precision and surface finish is a problem which has persisted in the art. A small deep, precise hole can be defined as having a diameter of less than 12 millimeters, aspect (depth/diameter) ratio of at least 5, precision of ISO standard H6-H7, angular tolerance of H6, surface roughness of 0.4-0.1 micrometer, and bore out-of-roundness, cylindrical out-of-roundness and taper which are within 1/2-1/3 of the tolerance.
Prior art methods for machining small, deep holes include drilling and expanding followed by rough and fine reaming, rough and fine boring, or boring and grinding. Other methods include honing and electron discharge machining (EDM). These prior art methods suffer from the drawbacks of multiple complex machining processes, extreme difficulty in obtaining satisfactory precision, surface finish and exchangeability, low productivity, poor quality control, high reject rate and often conical deformation at the exit ends of the holes.
Broaching is a process for machining holes, slots and grooves with high productivity compared to the methods described above. Broaching can be used for forming holes in numerous metals including low-carbon steel, low-carbon alloy steel, phosphor bronze, pure aluminum, stainless steel, titanium alloys and other materials.
A broaching tool generally includes an elongated body on which a number of parallel cutting teeth are formed or attached. The diameters of the teeth progressively increase from the front end to the rear end of the tool by an increment known as the "rise", such that each tooth cuts slightly deeper than the previous tooth.
A basic broaching tool and method are described in U.S. Pat. No. 1,945,535, entitled "BROACHING TOOL", issued Feb. 6, 1934 to B. Schlitz. A method of fabricating a basic broaching tool is described in U.S. Pat. No. 4,498,361, entitled "BROACH MANUFACTURING METHOD", issued Feb. 12, 1985 to W. Grace.
Broaching as practiced conventionally has not achieved its potential for forming small, deep holes with high precision and surface finish. This is due to a number of fundamental problems which have remained unsolved.
As the broaching tool is forced through the workpiece, high friction and specific pressure between the front face of each cutting tooth and the compressed material ahead of the tooth generate a large amount of heat which results in the formation of a layer of material which clings to the front face of the tooth and is known as a "built-up edge".
A broaching tool is preferably designed such that the built-up edge will attain a relatively small critical mass and then disintegrate or fracture off. If this action occurs, a smooth hole can be formed. A broaching tool which is specifically designed to minimize built-up edge formation is described in U.S. Pat. No. 2,392,481, entitled "MACHINE TOOL CUTTER", issued Jan. 8, 1946 to L. Kaplan et al.
Certain "sticky" materials such as stainless steel exhibit high elasticity, percentage elongation and plastic deformation. The frictional forces and pressures between the broaching tool and the workpiece are especially high for these materials, causing scaling of the surface of the hole and further enabling the built-up edge to grow to an undesirably large size. This causes the diameter of the hole to progressively increase, and creates a "nibbled" surface finish with a high degree of roughness.
If the built-up edge grows to a large size and then fractures off, the hole will have a surface with band-shaped scaling. Because cooling and lubrication are relatively ineffective in the lower portion of a deep hole which is being formed by vertical broaching, the scaling bands generally appear in the lower half of the hole.
Stainless steel is not only highly resilient and susceptible to plastic deformation, it also tends to be surface hardened by cutting. This causes the teeth in the rear portion of the broaching tool to slip, rather than to cut, increasing the difficulty of machining and degrading the surface quality.
The front face of each tooth has a "face angle", and the rear face or land of each tooth has a "backoff angle". The face and backoff angles are generally made as large as practical to increase the cutting efficiency and minimize heat generation. Prior art broaching tools are generally not guided smoothly through holes and tend to vibrate laterally, creating ring-shaped tool traces in the hole surfaces.
Another problem with sticky metals such as stainless steel is that built-up edges can also be formed between the lands of the cutting teeth and the surface of the hole. This will be exacerbated if the backoff angle is too small and/or the smoothness of the land is low. The built-up edges on the lands of the teeth fracture off unevenly, causing axial tool traces on the surfaces of the holes.
Prior art broaching tools have been formed with a rear "pilot" including a number of non-cutting rings having a diameter which is slightly smaller than the diameter of the finished hole. The purpose of the rear pilot is to smoothly guide the rear end of the broaching tool out of the hole. However, due to the clearance between the rings and the hole, the tool is able to move or vibrate radially, causing the exit end of the hole to be deformed into a conical shape. In extreme cases, the last ring of the rear pilot can nibble off a piece of metal from one side of the hole.
Prior art broaching tools for small, deep holes have conventionally been very long and thin. Fabrication of these tools is difficult, since they tend to bend during heat treatment and machining. A broaching tool which is not extremely straight cannot form a hole with high precision and surface finish.
Prior art lubricants and coolants based on conventional cutting oils are incapable of adequately reducing the frictional forces, temperatures and pressures created during broaching small, deep holes. This has heretofore limited the precision and surface finish of holes formed by broaching.